In the sensorless control device of a synchronous motor, many methods have been proposed as a restart method from a free-run state. A free-run state is a state in which a rail car is performing a coasting operation. The coasting operation is a state in which the notch command from a driver's seat is 0 and all switching element gate commands of an inverter are OFF.
Phase angle estimation methods may use induction voltage when vehicle speed is high, and may further involve a method of zero current control and a method of using a short-circuit current. Phase angle estimation methods may use inductance when vehicle speed is low. There is also a phase angle estimation method which chooses a suitable technique according to motor angular velocity using both the method using induction voltage and the method using inductance. By using these methods, starting from the state of a free run is possible.
Neither of the phase angle estimation methods can be started in a full speed region only by a single technique. Therefore, it is necessary to combine a plurality of phase angle estimation methods in the case of phase angle estimation. When phase angle estimation methods are combined in order to try starting with a plurality of phase estimation methods, the time which the reboot of an inverter takes becomes remarkably long in the worst case.
And, in order to use combined phase angle estimation methods, there is a problem of the starting sequence becoming complicated. The reboot time of an inverter is the time from when a torque (current) command occurs until the real angle and presumed angle of a motor rotor are in agreement and torque occurs on a motor. The reboot time greatly influences the response after giving a torque command. Usually, shortening the time in the worst case leads to improvement in a response.
In a system having a load contact machine between an inverter and a synchronous motor, it is possible to carry out a free run with a load contact machine in an open state when no-load induction voltage is more than the voltage of a direct current side of an inverter (a state hereinafter called a high-voltage state). In the case of the reboot of an inverter, the voltage of a direct current side of an inverter may turn into excess voltage from a free-run state with no-load induction voltage. Therefore, it is necessary to enable starting safely in the high-voltage state.